Inhibition of cysteine protease cathepsin Lincreases the level and activity of lysosomal glucocerebrosidase.

The glucocerebrosidase (GCase) encoded by the GBA1 gene hydrolyzes glucosylceramide (GluCer) to ceramide and glucose in lysosomes. Homozygous or compound heterozygous GBA1 mutations cause the lysosomal storage disease Gaucher disease (GD) due to severe loss of GCase activity. Loss-of-function variants in the GBA1 gene are also the most common genetic risk factor for Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Restoring lysosomal GCase activity represents an important therapeutic approach for GBA1-associated diseases. We hypothesized that increasing the stability of lysosomal GCase protein could correct deficient GCase activity in these conditions. However, it remains unknown how GCase stability is regulated in the lysosome. We found that cathepsin L, a lysosomal cysteine protease, cleaves GCase and regulates its stability. In support of these data, GCase protein was elevated in the brain of cathepsin L-KO mice. Chemical inhibition of cathepsin L increased both GCase levels and activity in fibroblasts from patients with GD. Importantly, inhibition of cathepsin L in dopaminergic neurons from a patient GBA1-PD led to increased GCase levels and activity as well as reduced phosphorylated α-synuclein. These results suggest that targeting cathepsin L-mediated GCase degradation represents a potential therapeutic strategy for GCase deficiency in PD and related disorders that exhibit decreased GCase activity.

BRASSINOSTEROID-SIGNALING KINASE1 associates with and is required for cysteine protease RESPONSE TO DEHYDRATION 19-mediated disease resistance in Arabidopsis.

The receptor-like cytoplasmic kinase BRASSINOSTEROID-SIGNALING KINASE1 (BSK1) interacts with pattern recognition receptor (PRR) FLAGELLIN SENSING2 (FLS2) and positively regulates plant innate immunity in Arabidopsis thaliana. However, the molecular components involved in BSK1-mediated immune signaling remain largely unknown. To further explore the molecular mechanism underlying BSK1-mediated disease resistance, we screened two cysteine proteases, RESPONSE TO DEHYDRATION 19 (RD19) and RD19-LIKE 2 (RDL2), as BSK1-binding partners. Overexpression of RD19, but not RDL2, displayed an autoimmune phenotype, presenting programmed cell death and enhanced resistance to multiple pathogens. Interestingly, RD19-mediated immune activation depends on BSK1, as knockout of BSK1 in RD19-overexpressing plants rescued their autoimmunity and abolished the increased resistance. Furthermore, we found that BSK1 plays a positive role in maintaining RD19 protein abundance in Arabidopsis. Our results provide new insights into BSK1-mediated immune signaling and reveal a potential mechanism by which BSK1 stabilizes RD19 to promote effective immune output.

XYLEM CYSTEINE PEPTIDASE 1 and its inhibitor CYSTATIN 6 regulate pattern-triggered immunity by modulating the stability of the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D.

Plants produce a burst of reactive oxygen species (ROS) after pathogen infection to successfully activate immune responses. During pattern-triggered immunity (PTI), ROS are primarily generated by the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD). RBOHD is degraded in the resting state to avoid inappropriate ROS production; however, the enzyme mediating RBOHD degradation and how to prevent RBOHD degradation after pathogen infection is unclear. In this study, we identified an Arabidopsis (Arabidopsis thaliana) vacuole-localized papain-like cysteine protease, XYLEM CYSTEINE PEPTIDASE 1 (XCP1), and its inhibitor CYSTATIN 6 (CYS6). Pathogen-associated molecular pattern-induced ROS burst and resistance were enhanced in the xcp1 mutant but were compromised in the cys6 mutant, indicating that XCP1 and CYS6 oppositely regulate PTI responses. Genetic and biochemical analyses revealed that CYS6 interacts with XCP1 and depends on XCP1 to enhance PTI. Further experiments showed that XCP1 interacts with RBOHD and accelerates RBOHD degradation in a vacuole-mediated manner. CYS6 inhibited the protease activity of XCP1 toward RBOHD, which is critical for RBOHD accumulation upon pathogen infection. As CYS6, XCP1, and RBOHD are conserved in all plant species tested, our findings suggest the existence of a conserved strategy to precisely regulate ROS production under different conditions by modulating the stability of RBOHD.

Applying the bioisosterism strategy to obtain lead compounds against SARS-CoV-2 cysteine proteases: An in-silico approach.

SARS-CoV-2 cysteine proteases are essential nonstructural proteins due to their role in the formation of the virus multiple enzyme replication-transcription complex. As a result, those functional proteins are extremely relevant targets in the development of a new drug candidate to fight COVID-19. Based on this fact and guided by the bioisosterism strategy, the present work has selected 126 out of 1050 ligands from DrugBank website. Subsequently, 831 chemical analogs containing bioisosteres, some of which became structurally simplified, were created using the MB-Isoster software, and molecular docking simulations were performed using AutoDock Vina. Finally, a study of physicochemical properties, along with pharmacokinetic profiles, was carried out through SwissADME and ADMETlab 2.0 platforms. The promising results obtained with the molecules encoded as DB00549_BI_005, DB04868_BI_003, DB11984_BI_002, DB12364_BI_006 and DB12805_BI_004 must be confirmed by molecular dynamics studies, followed by in vitro and in vivo empirical tests that ratify the advocated in-silico results.

Regulation of Atg8 membrane deconjugation by cysteine proteases in the malaria parasite Plasmodium berghei.

During macroautophagy, the Atg8 protein is conjugated to phosphatidylethanolamine (PE) in autophagic membranes. In Apicomplexan parasites, two cysteine proteases, Atg4 and ovarian tumor unit (Otu), have been identified to delipidate Atg8 to release this protein from membranes. Here, we investigated the role of cysteine proteases in Atg8 conjugation and deconjugation and found that the Plasmodium parasite consists of both activities. We successfully disrupted the genes individually; however, simultaneously, they were refractory to deletion and essential for parasite survival. Mutants lacking Atg4 and Otu showed normal blood and mosquito stage development. All mice infected with Otu KO sporozoites became patent; however, Atg4 KO sporozoites either failed to establish blood infection or showed delayed patency. Through in vitro and in vivo analysis, we found that Atg4 KO sporozoites invade and normally develop into early liver stages. However, nuclear and organelle differentiation was severely hampered during late stages and failed to mature into hepatic merozoites. We found a higher level of Atg8 in Atg4 KO parasites, and the deconjugation of Atg8 was hampered. We confirmed Otu localization on the apicoplast; however, parasites lacking Otu showed no visible developmental defects. Our data suggest that Atg4 is the primary deconjugating enzyme and that Otu cannot replace its function completely because it cleaves the peptide bond at the N-terminal side of glycine, thereby irreversibly inactivating Atg8 during its recycling. These findings highlight a role for the Atg8 deconjugation pathway in organelle biogenesis and maintenance of the homeostatic cellular balance.

C-terminal modification and functionalization of proteins via a self-cleavage tag triggered by a small molecule.

The precise modification or functionalization of the protein C-terminus is essential but full of challenges. Herein, a chemical approach to modify the C-terminus is developed by fusing a cysteine protease domain on the C-terminus of the protein of interest, which could achieve the non-enzymatic C-terminal functionalization by InsP6-triggered cysteine protease domain self-cleavage. This method demonstrates a highly efficient way to achieve protein C-terminal functionalization and is compatible with a wide range of amine-containing molecules and proteins. Additionally, a reversible C-terminal de-functionalization is found by incubating the C-terminal modified proteins with cysteine protease domain and InsP6, providing a tool for protein functionalization and de-functionalization. Last, various applications of protein C-terminal functionalization are provided in this work, as demonstrated by the site-specific assembly of nanobody drug conjugates, the construction of a bifunctional antibody, the C-terminal fluorescent labeling, and the C-terminal transpeptidation and glycosylation.

Remarkable Effects of a Rhenium(I)-diselenoether Drug on the Production of Cathepsins B and S by Macrophages and their Polarizations.

BACKGROUND/OBJECTIVE: Tumor-associated macrophages (TAMs) produce an excessive amount of cysteine proteases, and we aimed to study the effects of anticancer rhenium(I)-diselenoether (Re-diSe) on the production of cathepsins B and S by macrophages. We investigated the effect of Re-diSe on lipopolysaccharides (LPS) induced M1 macrophages, or by interleukin 6 (IL-6) induced M2 macrophages. METHODS: Non-stimulated or prestimulated murine Raw 264 or human THP-1 macrophages were exposed to increasing concentrations of the drug (5, 10, 20, 50 and 100 µM) and viability was assayed by the MTT assay. The amount of cysteine proteases was evaluated by ELISA tests, the number of M1 and M2 macrophages by the expression of CD80 or CD206 biomarkers. The binding of Re-diSe with GSH as a model thiol-containing protein was studied by mass spectrometry. RESULTS: A dose-dependent decrease in cathepsins B and S was observed in M1 macrophages. There was no effect in non-stimulated cells. The drug induced a dramatic dose-dependent increase in M1 expression in both cells, significantly decreased the M2 expression in Raw 264 and had no effect in non-stimulated macrophages. The binding of the Re atom with the thiols was clearly demonstrated. CONCLUSION: The increase in the number of M1 and a decrease in M2 macrophages treated by Re-diSe could be related to the decrease in cysteine proteases upon binding of their thiol residues with the Re atom.

Cysteine proteases are activated in sensitive Amaranthus palmeri populations upon treatment with herbicides inhibiting amino acid biosynthesis.

The herbicides glyphosate and pyrithiobac inhibit the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the aromatic amino acid biosynthetic pathway and acetolactate synthase (ALS) in the branched-chain amino acid biosynthetic pathway, respectively. Here we characterise the protease activity profiles of a sensitive (S), a glyphosate-resistant (GR) and a multiple-resistant (MR) population of Amaranthus palmeri in response to glyphosate and pyrithiobac. Amino acid accumulation and cysteine protease activities were induced with both herbicides in the S population and with pyrithiobac in the GR population, suggesting that the increase in cysteine proteases is responsible for the increased degradation of the available proteins and the observed increase in free amino acids. Herbicides did not induce any changes in the proteolytic activities in the populations with target-site resistance, indicating that this effect was only induced in sensitive plants.

African swine fever virus S273R protein antagonizes type I interferon production by interfering with TBK1 and IRF3 interaction.

African swine fever (ASF) is originally reported in East Africa as an acute hemorrhagic fever. African swine fever virus (ASFV) is a giant and complex DNA virus with icosahedral structure and encodes a variety of virulence factors to resist host innate immune response. S273R protein (pS273R), as a SUMO-1 specific cysteine protease, can affect viral packaging by cutting polymeric proteins. In this study, we found that pS273R was an important antagonistic viral factor that suppressed cGAS-STING-mediated type I interferon (IFN-I) production. A detailed analysis showed that pS273R inhibited IFN-I production by interacting with interferon regulatory factor 3 (IRF3). Subsequently, we showed that pS273R disrupted the association between TBK1 and IRF3, leading to the repressed IRF3 phosphorylation and dimerization. Deletion and point mutation analysis verified that pS273R impaired IFN-I production independent of its cysteine protease activity. These findings will help us further understand ASFV pathogenesis.

XCP1 cleaves Pathogenesis-related protein 1 into CAPE9 for systemic immunity in Arabidopsis.

Proteolytic activation of cytokines regulates immunity in diverse organisms. In animals, cysteine-dependent aspartate-specific proteases (caspases) play central roles in cytokine maturation. Although the proteolytic production of peptide cytokines is also essential for plant immunity, evidence for cysteine-dependent aspartate-specific proteases in regulating plant immunity is still limited. In this study, we found that the C-terminal proteolytic processing of a caspase-like substrate motif "CNYD" within Pathogenesis-related protein 1 (PR1) generates an immunomodulatory cytokine (CAPE9) in Arabidopsis. Salicylic acid enhances CNYD-targeted protease activity and the proteolytic release of CAPE9 from PR1 in Arabidopsis. This process involves a protease exhibiting caspase-like enzyme activity, identified as Xylem cysteine peptidase 1 (XCP1). XCP1 exhibits a calcium-modulated pH-activity profile and a comparable activity to human caspases. XCP1 is required to induce systemic immunity triggered by pathogen-associated molecular patterns. This work reveals XCP1 as a key protease for plant immunity, which produces the cytokine CAPE9 from the canonical salicylic acid signaling marker PR1 to activate systemic immunity.

The intersection between cysteine proteases, Ca2+ signalling and cancer cell apoptosis.

Apoptosis is a highly complex and regulated cell death pathway that safeguards the physiological balance between life and death. Over the past decade, the role of Ca2+ signalling in apoptosis and the mechanisms involved have become clearer. The initiation and execution of apoptosis is coordinated by three distinct groups of cysteines proteases: the caspase, calpain and cathepsin families. Beyond its physiological importance, the ability to evade apoptosis is a prominent hallmark of cancer cells. In this review, we will explore the involvement of Ca2+ in the regulation of caspase, calpain and cathepsin activity, and how the actions of these cysteine proteases alter intracellular Ca2+ handling during apoptosis. We will also explore how apoptosis resistance can be achieved in cancer cells through deregulation of cysteine proteases and remodelling of the Ca2+ signalling toolkit.

Inhibition of Cysteine Proteases via Thiol-Michael Addition Explains the Anti-SARS-CoV-2 and Bioactive Properties of Arteannuin B.

Artemisia annua is the plant that produces artemisinin, an endoperoxide-containing sesquiterpenoid used for the treatment of malaria. A. annua extracts, which contain other bioactive compounds, have been used to treat other diseases, including cancer and COVID-19, the disease caused by the virus SARS-CoV-2. In this study, a methyl ester derivative of arteannuin B was isolated when A. annua leaves were extracted with a 1:1 mixture of methanol and dichloromethane. This methyl ester was thought to be formed from the reaction between arteannuin B and the extracting solvent, which was supported by the fact that arteannuin B underwent 1,2-addition when it was dissolved in deuteromethanol. In contrast, in the presence of N-acetylcysteine methyl ester, a 1,4-addition (thiol-Michael reaction) occurred. Arteannuin B hindered the activity of the SARS CoV-2 main protease (nonstructural protein 5, NSP5), a cysteine protease, through time-dependent inhibition. The active site cysteine residue of NSP5 (cysteine-145) formed a covalent bond with arteannuin B as determined by mass spectrometry. In order to determine whether cysteine adduction by arteannuin B can inhibit the development of cancer cells, similar experiments were performed with caspase-8, the cysteine protease enzyme overexpressed in glioblastoma. Time-dependent inhibition and cysteine adduction assays suggested arteannuin B inhibits caspase-8 and adducts to the active site cysteine residue (cysteine-360), respectively. Overall, these results enhance our understanding of how A. annua possesses antiviral and cytotoxic activities.

Recombinant sugarcane cystatin CaneCPI-5 promotes osteogenic differentiation.

Cysteine proteases orchestrate bone remodeling, and are inhibited by cystatins. In reinforcing our hypothesis that exogenous and naturally obtained inhibitors of cysteine proteases (cystatins) act on bone remodeling, we decided to challenge osteoblasts with sugarcane-derived cystatin (CaneCPI-5) for up to 7 days. To this end, we investigated molecular issues related to the decisive, preliminary stages of osteoblast biology, such as adhesion, migration, proliferation, and differentiation. Our data showed that CaneCPI-5 negatively modulates both cofilin phosphorylation at Ser03, and the increase in cytoskeleton remodeling during the adhesion mechanism, possibly as a prerequisite to controlling cell proliferation and migration. This is mainly because CaneCPI-5 also caused the overexpression of the CDK2 gene, and greater migration of osteoblasts. Extracellular matrix remodeling was also evaluated in this study by investigating matrix metalloproteinase (MMP) activities. Our data showed that CaneCPI-5 overstimulates both MMP-2 and MMP-9 activities, and suggested that this cellular event could be related to osteoblast differentiation. Additionally, differentiation mechanisms were better evaluated by investigating Osterix and alkaline phosphatase (ALP) genes, and bone morphogenetic protein (BMP) signaling members. Altogether, our data showed that CaneCPI-5 can trigger biological mechanisms related to osteoblast differentiation, and broaden the perspectives for better exploring biotechnological approaches for bone disorders.

Origin and Early Diversification of the Papain Family of Cysteine Peptidases.

Peptidases of the papain family play a key role in protein degradation, regulated proteolysis, and the host-pathogen arms race. Although the papain family has been the subject of many studies, knowledge about its diversity, origin, and evolution in Eukaryota, Bacteria, and Archaea is limited; thus, we aimed to address these long-standing knowledge gaps. We traced the origin and expansion of the papain family with a phylogenomic analysis, using sequence data from numerous prokaryotic and eukaryotic proteomes, transcriptomes, and genomes. We identified the full complement of the papain family in all prokaryotic and eukaryotic lineages. Analysis of the papain family provided strong evidence for its early diversification in the ancestor of eukaryotes. We found that the papain family has undergone complex and dynamic evolution through numerous gene duplications, which produced eight eukaryotic ancestral paralogous C1A lineages during eukaryogenesis. Different evolutionary forces operated on C1A peptidases, including gene duplication, horizontal gene transfer, and gene loss. This study challenges the current understanding of the origin and evolution of the papain family and provides valuable insights into their early diversification. The findings of this comprehensive study provide guidelines for future structural and functional studies of the papain family.

Ethanol inhibits pancreatic acinar cell autophagy through upregulation of ATG4B, mediating pathological responses of alcoholic pancreatitis.

Excessive alcohol intake is a major risk factor for pancreatitis, sensitizing the exocrine pancreas to stressors by mechanisms that remain obscure. Impaired autophagy drives nonalcoholic pancreatitis, but the effects of ethanol (EtOH) and alcoholic pancreatitis on autophagy are poorly understood. Here, we find that ethanol reduces autophagosome formation in pancreatic acinar cells, both in a mouse model of alcoholic pancreatitis induced by a combination of EtOH diet and cerulein (a CCK ortholog) and in EtOH+CCK-treated acinar cells (ex vivo model). Ethanol treatments decreased pancreatic level of LC3-II, a key mediator of autophagosome formation. This was caused by ethanol-induced upregulation of ATG4B, a cysteine protease that, cell dependently, regulates the balance between cytosolic LC3-I and membrane-bound LC3-II. We show that ATG4B negatively regulates LC3-II in acinar cells subjected to EtOH treatments. Ethanol raised ATG4B level by inhibiting its degradation, enhanced ATG4B enzymatic activity, and strengthened its interaction with LC3-II. We also found an increase in ATG4B and impaired autophagy in a dissimilar, nonsecretagogue model of alcoholic pancreatitis induced by EtOH plus palmitoleic acid. Adenoviral ATG4B overexpression in acinar cells greatly reduced LC3-II and inhibited autophagy. Furthermore, it aggravated trypsinogen activation and necrosis, mimicking key responses of ex vivo alcoholic pancreatitis. Conversely, shRNA Atg4B knockdown enhanced autophagosome formation and alleviated ethanol-induced acinar cell damage. The results reveal a novel mechanism, whereby ethanol inhibits autophagosome formation and thus sensitizes pancreatitis, and a key role of ATG4B in ethanol's effects on autophagy. Enhancing pancreatic autophagy, particularly by downregulating ATG4B, could be beneficial in mitigating the severity of alcoholic pancreatitis.NEW & NOTEWORTHY Ethanol sensitizes mice and humans to pancreatitis, but the underlying mechanisms remain obscure. Autophagy is important for maintaining pancreatic acinar cell homeostasis, and its impairment drives pancreatitis. This study reveals a novel mechanism, whereby ethanol inhibits autophagosome formation through upregulating ATG4B, a key cysteine protease. ATG4B upregulation inhibits autophagy in acinar cells and aggravates pathological responses of experimental alcoholic pancreatitis. Enhancing pancreatic autophagy, particularly by down-regulating ATG4B, could be beneficial for treatment of alcoholic pancreatitis.

Crystal Structure of Staphopain C from Staphylococcus aureus.

Staphylococcus aureus is a common opportunistic pathogen of humans and livestock that causes a wide variety of infections. The success of S. aureus as a pathogen depends on the production of an array of virulence factors including cysteine proteases (staphopains)-major secreted proteases of certain strains of the bacterium. Here, we report the three-dimensional structure of staphopain C (ScpA2) of S. aureus, which shows the typical papain-like fold and uncovers a detailed molecular description of the active site. Because the protein is involved in the pathogenesis of a chicken disease, our work provides the foundation for inhibitor design and potential antimicrobial strategies against this pathogen.

Identification and classification of papain-like cysteine proteinases.

Papain-like cysteine peptidases form a big and highly diverse superfamily of proteins involved in many important biological functions, such as protein turnover, deubiquitination, tissue remodeling, blood clotting, virulence, defense, and cell wall remodeling. High sequence and structure diversity observed within these proteins hinders their comprehensive classification as well as the identification of new representatives. Moreover, in general protein databases, many families already classified as papain like lack details regarding their mechanism of action or biological function. Here, we use transitive remote homology searches and 3D modeling to newly classify 21 families to the papain-like cysteine peptidase superfamily. We attempt to predict their biological function and provide structural characterization of 89 protein clusters defined based on sequence similarity altogether spanning 106 papain-like families. Moreover, we systematically discuss observed diversity in sequences, structures, and catalytic sites. Eventually, we expand the list of human papain-related proteins by seven representatives, including dopamine receptor-interacting protein 1 as potential deubiquitinase, and centriole duplication regulating CEP76 as retaining catalytically active peptidase-like domain. The presented results not only provide structure-based rationales to already existing peptidase databases but also may inspire further experimental research focused on peptidase-related biological processes.

Iron restriction increases the expression of a cytotoxic cysteine proteinase TvCP2 by a novel mechanism of tvcp2 mRNA alternative polyadenylation in Trichomonas vaginalis.

Trichomonas vaginalis TvCP2 (TVAG_057000) is a cytotoxic cysteine proteinase (CP) expressed under iron-limited conditions. This work aimed to identify one of the mechanisms of tvcp2 gene expression regulation by iron at the posttranscriptional level. We checked tvcp2 mRNA stability under both iron-restricted (IR) and high iron (HI) conditions in the presence of actinomycin D. Greater stability of the tvcp2 mRNA under the IR than in HI conditions was observed, as expected. In silico analysis of the 3' regulatory region showed the presence of two putative polyadenylation signals in the tvcp2 transcript. By 3'-RACE assays, we demonstrated the existence of two isoforms of the tvcp2 mRNA with different 3'-UTR that resulted in more TvCP2 protein under IR than in HI conditions detected by WB assays. Additionally, we searched for homologs of the trichomonad polyadenylation machinery by an in silico analysis in the genome database, TrichDB. 16 genes that encode proteins that could be part of the trichomonad polyadenylation machinery were found. qRT-PCR assays showed that most of these genes were positively regulated by iron. Thus, our results show the presence of alternative polyadenylation as a novel iron posttranscriptional regulatory mechanism in T. vaginalis for the tvcp2 gene expression.

Brassica napus Drought-Induced 22-kD Protein (BnD22) Acts Simultaneously as a Cysteine Protease Inhibitor and Chlorophyll-Binding Protein.

Class II water-soluble chlorophyll proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins that bind with chlorophyll (Chl) and its derivatives. The physiological function of WSCPs is still unclear, but it is assumed to be involved in stress responses, which is likely related to their Chl-binding and protease inhibition (PI) activities. Yet, the dual function and simultaneous functionality of WSCPs must still be better understood. Here, the biochemical functions of Brassica napus drought-induced 22-kDa protein (BnD22), a major WSCP expressed in B. napus leaves, were investigated using recombinant hexahistidine-tagged protein. We showed that BnD22 inhibited cysteine proteases, such as papain, but not serine proteases. BnD22 was able to bind with Chla or Chlb to form tetrameric complexes. Unexpectedly, BnD22-Chl tetramer displays higher inhibition toward cysteine proteases, indicating (i) simultaneous Chl-binding and PI activities and (ii) Chl-dependent activation of PI activity of BnD22. Moreover, the photostability of BnD22-Chl tetramer was reduced upon binding with the protease. Using three-dimensional structural modeling and molecular docking, we revealed that Chl binding favors interaction between BnD22 and proteases. Despite its Chl-binding ability, the BnD22 was not detected in chloroplasts but rather in the endoplasmic reticulum and vacuole. In addition, the C-terminal extension peptide of BnD22, which cleaved off post-translationally in vivo, was not implicated in subcellular localization. Instead, it drastically promoted the expression, solubility and stability of the recombinant protein.

Legumain Functions as a Transient TrkB Sheddase.

While primarily found in endo-lysosomal compartments, the cysteine protease legumain can also translocate to the cell surface if stabilized by the interaction with the RGD-dependent integrin receptor αVß3. Previously, it has been shown that legumain expression is inversely related to BDNF-TrkB activity. Here we show that legumain can conversely act on TrkB-BDNF by processing the C-terminal linker region of the TrkB ectodomain in vitro. Importantly, when in complex with BDNF, TrkB was not cleaved by legumain. Legumain-processed TrkB was still able to bind BDNF, suggesting a potential scavenger function of soluble TrkB towards BDNF. The work thus presents another mechanistic link explaining the reciprocal TrkB signaling and δ-secretase activity of legumain, with relevance for neurodegeneration.